Agents that are effective in killing neoplastic and other diseased or abnormal cells generally cannot be administered to a patient in effective doses because they also exert cytotoxic effects on normal cells. Therapeutic protocols for treating cancer and other disorders using cytotoxic agents such as toxins, drugs, radioisotopes, and the like are generally limited by the toxicity of the cytotoxic agent to normal cells and tissues. Efforts have therefore been directed to linking cytotoxic therapeutic agents to targeting agents, such as antibodies, which are capable of localizing at certain target cells and tissue(s).
Research efforts in the field of tumor immunology have identified antibodies to antigenic determinants expressed preferentially on tumor cells. Such antibodies, and fragments thereof, may be employed as carriers for cytotoxic agents to provide selective delivery of cytotoxic agents to target tissues. Antibodies, fragments thereof, and the like have also been utilized as carriers for diagnostic agents such as diagnostic radioisotopes to provide selective delivery of the diagnostic agent to target tissues, thereby providing enhanced diagnostic imaging properties. Diagnostic and therapeutic immunoconjugates comprising an active moiety exhibiting diagnostic or therapeutic properties and a targeting moiety exhibiting specificity and affinity for target cells, tissue(s), antigens, or the like, are believed to be of tremendous potential in diagnosis and treatment of cancer and a variety of other diseases. Development of techniques for generating monoclonal antibodies having specificity for a single epitope has further expanded the potential for immunoconjugates as in vivo diagnostic and therapeutic agents.
Most antibodies capable of localizing at human tumors have some normal tissue cross-reactivity. In general, cross-reactive binding is characterized as binding of an antibody binding site at an epitope recognized by the antibody on non-target cells expressing the epitope. These cross-reactive sites may preferentially bind injected immunoconjugates or metabolites thereof. Cross-reactive, non-target binding may divert a substantial portion of the administered dose from the target site(s), especially if the cross-reactive, non-target sites are concentrated in well-perfused organs. Reducing cross-reactive binding of the antibody or conjugates to non-target cells without adversely affecting their tumor localization would be advantageous.
Nonspecific binding of an antibody or fragment or conjugate thereof generally occurs through mechanisms other than the antigen-recognition binding site on the antibody. For example, antibody may be bound to non-target sites in the liver and spleen when its F.sub.c receptors bind to cells in these non-target organs. Like cross-reactive binding, nonspecific binding may substantially reduce the target:nontarget ratio of immunoconjugate protocols, and reducing nonspecific binding of conjugates or metabolites thereof to non-target cells without adversely affecting their tumor localization would be advantageous.
Localization of specific antibodies, fragments, immunoconjugates and metabolites thereof at non-target sites due to nonspecific and cross-reactive binding has generally been dealt with, if at all, by co-administering relatively large doses of specific, unconjugated antibody. U.S. patent application No. 06/917,176, filed Oct. 9, 1986, now abandoned, teaches a method for enhanced delivery of immunoconjugates to target cells comprising administering an adequate dosage of blocking antibodies or fragments capable of nonspecific and/or cross-reactive binding to non-target cells, in addition to an effective dosage of immunoconjugates comprising antibodies or fragments specific for the target cells. Irrelevant antibodies may be used to reduce nonspecific and cross-reactive binding of specific antibodies, and unconjugated specific antibody may be used to bind cross-reactive sites prior to the administration of the conjugated specific antibody.
Antibody fragments are frequently used as the targeting moiety for immunoconjugates. Experimental evidence has suggested that immunoconjugates comprising antibody fragments accumulate at target sites such as tumor sites more rapidly than immunoconjugates comprising their whole antibody counterparts. More rapid target site accumulation may be due to the smaller size of the targeting moiety, since decreased immunoconjugate size generally facilitates egress across the blood vessel and capillary walls into the tumor bed. Antibody fragments have shorter serum half-lives than the corresponding whole antibody, however, and a relatively large percentage of the immunoconjugates comprising antibody fragments may be cleared from circulation prior to tumor localization, despite the increased tumor localization capability of the fragments.
Reduction of immunogenicity is yet another important factor in immunoconjugate development efforts. Where the targeting moiety of the immunoconjugate is derived from a species different from that of the patient, the likelihood of stimulating antiglobulin production in the patient, particularly after repeated diagnostic and/or therapeutic treatments, is quite high. In efforts to reduce immunogenicity, antibody fragments, including various constituents of the antibody variable regions, have been used to quantitatively reduce the amount of antibody present. Use of antibody fragments as targeting moieties in immunoconjugates introduces other problems, however, such as reduced serum half-life and more rapid clearance.